The present invention relates to systems and methods for controlling a physical characteristic of a manufactured material simultaneously with the manufacturing process. More particularly, the present invention relates to a moisture control system and method which control the moisture content profile of paper sheet, in substantially discrete longitudinal segments, or "slices."
A typical paper manufacturing system has a "wet" end where its manufacturing process begins, and a "dry" end where the final product is rolled into large reels. The dry end is defined as being "downstream" from the wet end, and the wet end "upstream" from the dry end. At the wet end, a fluid, fibrous pulp mixture, referred to as "slurry," flows from a headbox onto a swiftly moving, continuous, porous belt referred to as the "wire." This forms a continuous moving sheet of slurry, referred to as the "web," which begins to lose its moisture by drainage through the pores in the wire. The web continues downstream where, prior to leaving the wire, it passes over a vacuum box which draws more moisture from the sheet. The vacuum within the vacuum box may be generated in a number of ways well known in the art; for example, by drawing air from within the vacuum box through an exhaust port with a fan.
To assist the vacuum box in drawing moisture from the web, steam may be applied to the top surface of the web by way of a "steam profiling bar" prior to the web passing over the vacuum box. By applying steam to the top surface of the web, the moisture within the web is heated and therefore its viscosity is reduced. This reduced viscosity causes the moisture to be more easily withdrawn from the web by the vacuum box. The steam profiling bar is capable of applying steam in a non-continuous manner across the width of the web. In other words, separate steam applicators within the steam profiling bar apply steam to substantially discrete, but slightly overlapping longitudinal segments, or "slices," across the width, or "cross-direction," of the web. Thus, more or less steam may be applied among the individual slices so that more or less moisture may be withdrawn from the individual slices, thereby allowing the moisture content profile in the cross-direction of the web to be tailored as desired.
Following the steam profiling bar and vacuum box, the continuous moving sheet leaves the wire and typically enters a series of opposing, felt-covered rolls which press additional moisture from the sheet. These rolls form the "press section" of the paper making machine. Following the press section, the sheet goes through a series of heated drying drums. As the sheet winds through these drying drums, it maintains physical contact with much of the circumferential surface area of each drying drum. This allows the drying drums, which are filled with pressurized steam, to heat and therefore still further dry the sheet.
Part way through this series of drying drums, the sheet passes a "moisture profiling bar." This moisture profiling bar sprays the sheet with water as desired across the cross-direction of the sheet. Similar to the steam profiling bar discussed above, this moisture profiling bar is capable of applying varying amounts of water to each slice across the cross-direction of the sheet, thereby allowing the moisture content profile in the cross-direction to be tailored as desired.
Following the moisture profiling bar, the last of the drying drums, and possibly additional processing, the sheet is rolled into a large reel. Immediately prior to being rolled up into the reel, the sheet will typically pass a scanner containing several sensors. These sensors are moved back and forth across the width of the sheet by the scanner and monitor various physical characteristics of the sheet, such as gloss, density and moisture content. Measuring the moisture content profile at this point indicates whether the paper just produced is either too wet or too dry. These measurements may be compared against the desired moisture content profile, whereupon the steam applied by the steam profiling bar and water applied by the moisture profiling bar may be adjusted appropriately.
The purpose of the steam and moisture profiling bars is to control the final moisture content profile of the manufactured paper. Typically, the desired profile will be flat. That is, it is usually desirable to have a uniform moisture content across the width and length of the sheet. Paper is sold by weight and generally has a moisture content profile specification. If the moisture content profile can be tightly controlled, the paper can be produced having the maximum moisture content, while still being within the specified limits. This results in a cheaper product, since the final product for sale contains less pulp and chemicals (expensive) and more water (inexpensive) per pound.
The prior art uses several means by which the moisture and steam profiling bars are controlled to achieve and maintain the desired moisture content. One means uses the measurements from the single scanner as discussed above to control both the moisture profiling bar and the steam profiling bar. In an attempt to avoid interaction between the moisture and steam profiling bars, i.e., applying both moisture and steam to the same portion of paper sheet, the control loops for the two profiling bars are "de-tuned." In other words, the control loops are established such that one profiling bar would respond substantially more slowly than the other profiling bar. However, this means is not entirely successful since both water and steam may still be applied to the same portions of the sheet. This is undesirable since the application of water would tend to nullify the effect of the application of the steam. Moreover, steam is relatively expensive to produce and apply, and such an expense is simply wasted when the drying effects of the steam are nullified by the subsequent application of water.
A second prior art system provides for the use of a second scanner associated with the steam profiling bar, whereby each of the profiling bars would thus have its own dedicated scanner. This second scanner would be located at a point between the steam profiling bar and the moisture profiling bar. Each scanner would independently control its own profiling bar. This configuration, however, also may result in the application of both water and steam to the same slice. This is undesirable for the reasons stated above, and moreover, requires the added expense and maintenance costs of a second scanner.